Microsoft has announced that customers with Microsoft 365 Enterprise licenses will also be migrated to the new Outlook from 2026. In contrast to "classic Outlook", the "new Outlook version" is a web app. A corresponding announcement was published today on Heise Newsletter has been published. However, the message can also be misunderstood somewhat, because this is not about on-prem Exchange customers with classic Outlook, but about Microsoft 365 users who already use Exchange Online.
Here is the original message from Microsoft from the M365 Admin Center translated into German:
We are making some changes to the migration from classic Outlook to the new Outlook for Windows.
Our aim is to provide more than 12 months' notice of these changes so that our customers can prepare for them. From April 2026, users with Microsoft 365 for Enterprise licenses will be migrated from classic Outlook for Windows to the new Outlook for Windows. As part of this rollout, users will be migrated to the new Outlook once, with the option to be migrated again in the future. However, users will retain the option to switch back to and use classic Outlook.
With this change, we want to give users the opportunity to try out the new Outlook, as millions of users have already done. The new Outlook offers the most advanced user experience with features like Copilot, customizable themes, and a variety of time-saving features like pinning and resetting emails. Users are also invited to give us feedback on the new Outlook via the feedback feature in the help section so we can provide the best email and calendar experience.
Summary:
From April 2026, Microsoft 365 for Enterprise users will be migrated from classic Outlook to the new Outlook for Windows, with the option to revert back to classic Outlook. This change aims to improve the user experience with modern features. Organizations will be informed to prepare, with the automatic switch requiring no administrator action but offering opt-out options.
Source: Microsoft 365 Admin Portal
Here you can find the article for the opt-out or for controlling the Outlook migration:
It is important to know that this is about the conversion of cloud customers, not about customers who use Exchange on-prem with the classic Office package. They will continue to use the classic Outlook (which is also part of Office 2024). I have received a few emails about this changeover, as the new Outlook Web App runs on the local computer but processes the data in the Microsoft Cloud. Heise writes the following:
New Outlook may be problematic
The new Outlook relies on server-side mail processing and therefore collects all emails in Microsoft's cloud. For example, the new Outlook transfers IMAP access data to Microsoft servers, which then collect emails from there. The new functions, such as AI support, rely on server-side computing power. This can be undesirable, especially in a corporate environment. Another frequent problem is that the new Outlook cannot handle local mailbox files, such as .pst files.
Source: Timetable: Microsoft wants to migrate enterprise customers to the new Outlook in 2026
This is of course correct, the new Outlook web app transfers the access data to Microsoft. However, if Microsoft 365 (including Exchange Online) is already in use, then this access data, as well as the mailbox data, is usually already stored by Microsoft anyway. IMAP accounts and PST files are less likely to be used here.
It is therefore correct that M365 customers will receive the new Outlook Web App from April 2026. The changeover can be controlled. The access data will then be with Microsoft (in most cases they already are). For on-prem customers with classic Outlook, nothing will change for the time being. I hope this clears up some of the confusion that has arisen, unfortunately Microsoft doesn't make it easy with the same product names for different versions.